drm/i915: Deprecated UMS support

drm/i915: Deprecated UMS support

It's been 5 years since kms support was merged and roughly 4 years
since UMS support was ripped out from userspace drivers.

Thus far it's not been a big burden to keep the ums paths alive, and
we've made some good progress in better separating it from the kms
code by sprinkling DRIVER_MODESET checks all over the place.

But now that the drm demidlayering is within reach this changes. I
want to make the driver loading code more robust using devres.c and
other cool tricks. But that doesn't work with ums due to the
shadow-attach trick. Which means we either
a) need to split out a complete ums codebase like radeon has
b) kill it for good.

The 2nd option is obviously much less work than the first, so I think
it's time to test the waters and see how many people out there still
use ums.

I've decided that silently failing to initialize the driver (and not
e.g. failing to load the module) is the right thing. That way we
should only get reports from users that actually care about some ums
features (like accelerated gl or support for secondary outputs).
Everyone else will just fall back to the vesa X driver.

For developers there's a small info level dmesg output.

The plan is to drop this Kconfig option after 3.16 (so gives us 2 full
releases) and then start killing code for real 2-3 releases
afterwards. That should be more than enough time for users to pipe up.

Of course if anyone does we need to revisit this plan and maybe go
with option a) above.

Also enable the KMS support by default in Kconfig and polish the help
texts a bit.

v2: Add the missing hunk of actual code changes. Oops. (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Dave Airlie <airlied@gmail.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>